The present invention relates to an anaerobic digestor which decomposes organic material to produce a liquid fertilizer and methane gas, and in particular to an anaerobic digestor heated by solar energy.
It is well known to produce methane gas through the anaerobic decomposition of organic material or wastes containing methanogenic bacteria. The usual process is to combine the organic materials in a sealed container in which is maintained an anaerobic atmosphere and a constant temperature in the range of 90-95.degree. F. A period of approximately 30 days is required for an initial incubation before a sufficient quantity of gas withdrawn. A continuous process can be established whereby the feedstock of organic material is continually fed into the tank, and the expired effluent, a liquid fertilizer, is continually removed from the tank. The gas collected can be compressed and used as a methane energy source.
A problem inherent in the process exists in keeping the temperature at a constant value in the range of 90-95.degree. F., as required for the growth of the methanogenic bacteria. In mild climates some of the methane gas produced by the process can be used to heat the digestor, however in colder climates this is not efficient since a large proportion of the methane is used in the heating, leaving a low net yield of methane. Solar energy can be used to supplement the gas heating of the digestor in order to maintain an efficient operation.
Verani, in U.S. Pat. No. 3,933,328 describes an anaerobic digestor which is solar heated. The digestor is buried in the ground and covered with a liquid filled pond. The liquid, being absorptive of solar energy, is circulated through the digestor to heat the contents. A translucent roof, in the form of a dome or inflated bubble exterior of the pond, is used to establish a regulatory temperature environment.
Boblitz, in U.S. PAT. No. 4,057,401, provides a solar heated digestor which comprises a series of sealed containers surrounded with crushed stones, enclosed in a large chamber. The roof over the chamber is pivotal to be inclined at an angle to receive the sun's rays. A black wire screen covered with transparent material is positioned in the roof to absorb the solar energy, thereby heating the air in the roof which in turn is circulated around the sealed tanks.
The structures of Verani and Boblitz, although presumably efficient in using the solar radiation during the sunlight hours to heat the digestors, do not provide adequate restriction to heat loss to the outside environment during the non-sunlight hours, which in cold climates would allow the temperature within the digestor to fluctuate considerably.